Apatite-based glass-ceramics have long attracted interest as synthetic materials for bone replacement [1-5]. They are used in bulk form, powders, coatings, and more recently have been investigated as macroporous scaffolds [6]. The excellent biocompatibility of hydroxyapatite and fluorapatite glass-ceramics is classically related to their chemical and crystallographical similarities with the apatite phase present in bone [7]. Compared to bioinert ceramics such as alumina, the main potential advantage of apatite-based glass-ceramics is the formation of a chemical bond at the ceramic-bone interface [8, 9]. High resolution transmission electron microscopy (TEM) studies have clearly demonstrated the formation of de novo apatite crystals by epitaxial growth on the surface of hydroxyapatite-containing ceramics [10]. Moreover, apatite crystallization in apatite-mullite glass-ceramics has been shown to elicit an excellent bone tissue response after implantation in rat femurs, while the corresponding amorphous glass induced an inflammatory response. [11]
These findings raise the important issue of the role of topography and microstructural features in the pace of in vivo integration of apatite-based glass-ceramics and implant materials [12-14]. Meanwhile, previous work has revealed that fluorapatite glass-ceramics doped with small amounts of niobium oxide crystallized into a very fine dual microstructure composed of submicrometer fluorapatite spherical crystals, together with forsterite polygonal crystals [15]. This microstructure is strongly influenced by the conditions of crystallization heat treatment, namely duration, temperature and cooling rate [16]. Further work revealed that the surface topography associated with this type of microstructure led to excellent attachment, proliferation and differentiation of human mesenchymal stem cells [17]. Recent investigations on the crystallization mechanisms of apatite-mullite glass-ceramics also demonstrated that control of crystal morphology to form arrays of apatite nanocrystals is achievable in this system, through modulations of the glass composition and heat treatment regime [18-20].
Inspired by progress in the fabrication of open-celled ceramics, several processing techniques have been developed to prepare macroporous ceramic scaffolds for bone replacement [21]. Amongst these techniques, one of the most common is the impregnation of a open-cell polymer foam with a ceramic slurry that is later dried and sintered while the polymeric template is eliminated [22]. This polymer foam impregnation technique is an attractive method for producing glass-ceramic scaffolds from bioactive compositions, including hydroxyapatite, fluorapatite and β-tricalcium phosphate (β-TCP)-containing glass-ceramics [23]. However, hydroxyapatite and fluorapatite ceramics are traditionally difficult to sinter, even as mixtures of powders [24-26]. Low temperatures result in high porosity and incomplete sintering, while high temperatures in excess of 1000° C. may lead to decomposition, loss of hydroxyls or fluorine and formation of pyrophosphates [27]. Additionally, in glass-ceramic systems, crystallization may occur during sintering and hinder the densification process [28, 29].
Indeed, it is well established that independently of the nature of the crystalline phases forming, chemical compositional changes in the remaining glassy matrix are likely to induce changes in viscosity, which in turn may prevent adequate sintering [30-32]. Concurrently, several studies have shown that adequate sintering is only possible if sintering precedes crystallization [31, 33].
One way to improve sinterability for a given composition is to extend the working range to allow viscous flow sintering prior to crystallization. This can be done by fine-tuning the glass composition and replacing intermediate oxides such as alumina with alkaline-earth modifiers such as calcium oxide [34, 35]. Moreover, studies in multicomponent bioactive silicate glasses revealed that, when introduced as calcium fluoride, and in the presence of phosphorous pentoxide, calcium causes a decrease in the glass transition temperature, together with an increase in the crystallization temperature, thereby efficiently increasing the processing window [36]. Calcium is also a key component in the development of bioactivity in bioactive glasses [37]. Meanwhile, aluminum oxide has been shown to be detrimental to the bioactivity of calcium silicate glasses [38-40].